The "boundary line" tool that you found, can draw solid lines, and you can configure the line width and the colour. Unfortunately, this tool doesn't have an option to draw dashed lines.

Of course, you can draw each individual dash as a separate line, but this is a bit cumbersome.

Alternatively, you can configure a course leg line between two controls as a marked route. This makes the line dashed all the way from one control to the next. Double click on the course leg line to configue it.

I have put on the wish list for a future release an item to add a "dashed line" tool.